This invention relates to an apparatus for non-destructive ultrasonic testing such as detection of flaws or thickness measurement of a material. This invention has particular importance in the testing of elongated materials such as pipes and rods.
Application of a voltage pulse to a piezoelectric crystal will cause it to mechanically oscillate at its resonant frequency. Sound, at the resonant frequency of the crystal will, when it strikes the crystal, also cause oscillation. When the crystal oscillates, it generates a sinusodial voltage and a high frequency sound wave, both of which occur at the crystal's resonant frequency.
In ultrasonic flaw-detector and thickness-measuring instruments, a piezoelectric crystal (transducer) is placed on or near the surface of the body whose integrity or thickness is to be measured. In order to insure effective coupling between the transducer and the surface, the space between may be filled with an acoustically transparent material, i.e., a material having a small amount of accoustical attenuation such as water or oil. The output of a pulse generator, consisting of short duration voltage pulses, is applied to the crystal. In thickness-measuring instruments, high frequency sound generated by the crystal when it is pulsed passes through the body, is reflected from the opposite surface, and returns to the crystal where the back-reflected sound causes the crystal to again oscillate. The same sequence of events may happen repeatedly and there may be a second, third or even greater number of back-reflections due to the same voltage pulse. The voltage pulse initiating this sequence is called the initial pulse. By measuring the elapsed time between the initial pulse and a back-reflection or, between two back-reflections, and knowing the velocity of sound through the body being tested, the thickness can be determined. Similar technique is used in flaw detection.
Ultrasonic pulse-echo thickness-measurement apparatus generally consists of a highly damped piezoelectric transducer excited by an ultrasonic pulse generator connected thereto. Ultrasonic pulses of short duration are injected into a specimen such as a plate metal to determine the thickness D thereof. After entering the specimen, the ultrasonic pulse is repeatedly reflected back and forth between the parallel surfaces of the specimen separated by the dimension D until its energy is dissipated. During this reverberation process, piezoelectric transducer (which also acts as an ultrasonic receiver) generates a short voltage pulse each time the ultrasonic pulse strikes upon the specimen surface to which the piezoelectric transducer is coupled. Thus, following the emission of the initial excitation pulse, a sequence of electrical pulses is produced by the piezoelectric transducer. The time interval T between two consecutive pulses of this sequence is equivalent to the specimen thickness according to EQU T = 2D/V.sub.L
where V.sub.L represents the longitudinal ultrasonic wave velocity in the material of the specimen. Other waves such as shear waves can sometimes also be used. For a specific material, the longitudinal ultrasonic wave velocity is usually constant within a wide range of ultrasonic frequencies and the specimen thickness D can be determined by measuring the pulse period T, or its reciprocal.
The time interval between a pulse and its back-reflection or between various reflection pulses can be determined by displaying on an oscilloscope the sinusoidal voltage across the crystal corresponding to the pulse and back-reflections. Thickness of the body being tested can then be read on the horizontal or time axis of the oscilloscope. A more recent development is the direct-reading instrument which displays thickness measurements directly on a meter or on a digital read-out display. In the direct-reading instrument, a constant current source is used to charge a capacitor at a linear rate with respect to time. The constant current source is gated-on by the initial pulse and gated-off by the first back-reflection. The charge on the capacitor is, therefore, dependent on the elapsed time between the initial pulse and the first back-reflection which, in turn, depends on the thickness of the body. The charge on the capacitor at any time is indicated on a meter or a digital-type display. The readout, whether meter or digital-type, is calibrated directly, for example, in inches. Other means are available for measuring this time interval.
U.S. Pat. No. 3,557,610 provides an apparatus for the ultrasonic testing of tubes and other elongated bodies comprising a plurality of ultrasonic transducers, an amplifier for amplifying signals produced by said transducers when ultrasonic waves fall upon them, sequence control means for permitting passage of signals from said transducers to the amplifier in a cyclic sequence and automatic gain control means for the amplifier incorporating a plurality of pre-settable circuits operable sequentially in synchronism with said sequence control means. Unfortunately, this series of fixed transducers is expensive and does not completely test the circumference of the pipe.
In order to achieve more speed in testing, U.S. Pat. No. 3,415,111 provides a rotary transducer which is capable of moving the transducer elements around the surface of the tubes or rods being tested, while the tube is passed axially thereby. This apparatus comprises a head having therethrough a passage which the test piece is passed axially and a bore parellel to said passage, at least one elongated carrier member received by said bore in the head with its axis parallel to the axis of the passage extending through the head, a crystal transducer element mounted in a recess in said carrier member intermediate to the ends thereof, said crystal transducer element being arranged with the axis normal to its active surface being transverse to the axis of the carrier member, said carrier member being rotatable within the bore for selecting the direction of the normal axis of the crystal transducer element, an ultrasonic pulse generating and receiving instrument, and a pair of coupling means respectively on the carrier member and within said bore in the head electrically coupling the crystal transducer element to said ultrasonic pulse generating and receiving instrument. Unfortunately, the water acting as an accoustical transmitting medium is held within the testing apparatus by seals which protect bearing from water. These seals are subject to wear and can require substantial maintenance. Also, this type of instrument is not made for immersion in an accoustical medium, such as, for example, a water cooling bath in an extrusion line.
U.S. Pat. No. 3,885,419 teaches a method and apparatus for non-destructive testing of pipes and rods. Several test units in one plane provide right angles to the direction of predominant extension of the object to be tested. Relative rotation is provided between the test units and the pipe or rod to be tested, and on the axis of the latter. The units are alternatingly operated as transmitter and as receiver for ultrasonic pulses, while at least some of the units operate sequentially as transmitter for transmitting ultrasonic pulses in similar directions and into the test object as far as azimuthal component of propagation is concerned. A unit not transmitting in any instant operates as receiver which is particularly angularly displaced from a unit that operates as transmitter in that instant, but will transmit pulses in the same direction when operated as transmitter, then cooperating with another unit displaced further in the same direction and then operating as receiver, etc. This patent does not seem to discuss the problem of dealing with the accoustical medium, that is, containing the medium by seals.
It is an object of this invention to provide a method and apparatus for the rapid ultrasonic detection of flaws and thickness of elongated bodies such as tubes and rods or coatings thereon.
It is an object of this invention to provide a reliable rotary transducer mount for ultrasonic testing which does not contain seals for containing an accoustical medium within.
It is further an object of this invention to provide a rapid ultrasonic detection device for elongated bodies which can be immersed in an acoustical transmitting liquid such as water cooling bath in a polymeric pipe extrusion line.
It is still further an object of this invention to provide an ultasonic testing device which can rapidly test the entire circumference of an elongated body.